Typical gaseous reduction systems incorporating vertical shaft, moving bed iron ore reduction reactors, are disclosed in U.S. Pat. Nos. 3,765,872; 3,770,421; 3,779,741; and 3,816,102. In such systems reduction of the ore has commonly been effected by a reducing gas largely composed of carbon monoxide and hydrogen obtained by catalytic reformation of a mixture of natural gas and steam. Such systems, typically, comprise a vertical shaft reactor having a reducing zone in the upper portion thereof and a cooling zone in the lower portion thereof. The ore to be reduced is fed to the top of the reactor and flows downwardly therethrough, first through the reducing zone wherein it is brought into contact with hot reducing gas and then through a cooling zone wherein it is cooled by a gaseous coolant before being removed at the bottom of the reactor. Effluent gas from the reducing zone is cooled to remove water therefrom and in most cases, a major part of the cooled effluent gas is reheated and recycled to the reducing zone. Similarly, at least a part of the coolant gas withdrawn from the cooling zone is commonly cooled and recycled to the cooling zone to cool the sponge iron to, or close to, the ambient temperature. At its lower end the reactor is provided with some means for controlling the discharge of the cooled sponge iron from the reactor, e.g. a rotary discharge valve, a vibratory chute, conveyor belt, or the like.
It is often desirable to discharge the sponge iron at high temperature for direct delivery to a steel refining furnace or, when the hot sponge iron cannot be used immediately, then to a melter or to a machine for briquetting the hot sponge iron product produced in such a reactor. Direct reduced iron in the form of briquettes is easier to handle and transport than when it is in the form of the more porous, friable, and smaller sponge iron particles. Also, briquetting of the sponge iron reduces the tendency to re-oxidize when stored in contact with atmospheric air. However, as noted above, the conventional reactors deliver the sponge iron at a temperature at or near ambient temperature. Densification of the product by briquetting at such low temperatures is difficult to achieve, requires high pressures, tends to produce friable products, and an undesirably high proportion of fines. As a practical matter the sponge iron should be relatively near the sintering temperature to produce briquettes having acceptable physical properties.
The production of sponge iron at a temperature suitable for hot briquetting is not just a matter of operating the cooling zone of a conventional reactor at a higher temperature. The cooling zone of the reactor performs not only a cooling function, but also serves as a carburizing zone to bring the sponge iron to a predetermined desired carbon content, which is preferably in the form of ferric carbide rather than elemental carbon (such as soot). With natural gas being used, as one of the possible sources of carbon for carburizing the sponge iron, the natural gas and the conditions within the cooling zone must be so maintained as to give the degree of carburization needed for the desired sponge iron product. Typically, carburization of from 1% to 4% is desired. In a non-stoichiometric reformer of the type shown in U.S. Pat. No. 4,370,162, the maximum obtainable carburization of sponge iron from the reducing zone would be about 0.5%.
Additionally, a briquetting of the hot sponge iron, preferably, should be done at above about 700.degree. C. Below this temperature for most types of sponge iron the pressures typically required are too great and the wear of the briquetting equipment becomes excessive. Also, it becomes too difficult to densify the sponge iron adequately. A briquette with a density of at least 5 gm/cm.sup.3 is desirable for convenience of handling and for passivation. Dense briquettes have decreased exposed surface, lower porosity, and are less prone to fragmentation and fines generation.